Seikanashi's Absurd Escapade: Embers of Legacy

One peaceful day, with not much happening. Sei and his friends are sucked into a world stuck in between the multiverse. They team up with alternate versions of themselves, new characters from other worlds, and a mysterious young girl who seems to not remember much.

Will they escape? Who is this young girl? Will Sei ever figure out how to counter Mars's obnoxious fighting game habits? And more importantly...

Is there something bigger at play?